Recent soaring crude oil prices economically allows development of deep oil well or wells which produces oil with the inferior quality such as sour crude. As a material of the oil well tubular used for drilling these oil wells, it is demanded that a stainless steel and/or a high-alloy steel these of which have higher mechanical strength than a normal carbon steel and/or have an excellent corrosion resistance to acidic component. Therefore, not only in the conventional extrusion working but also in Mannesmann pipe production, seamless pipe production using the stainless steel and the high-alloy steel is demanded.
However, compared with extrusion working such as Eugene pipe production method, Mannesmann pipe production method severely deforms the material, which results in generation of Mannesmann-specific flaws on the inner and outer surface of the pipe product. Especially, the outer surface flaws generated during the piercing-rolling step is one of the major causes of reduction of productivity. The outer surface flaw can be categorized into the following two types depending on the way of generation.
The first type of the flaw is the one which cannot be removed even by certain treatment (hereinafter, referred to as “flaw which cannot be removed”), it is caused by seizure between disk guides and a billet. It should be noted that the disk guide is classified with a fixed “disk shoe” and a “disk roll” which rotates itself while restraining outer circumference of the billet to de-escalate relative velocity to the billet surface. Below, the description will be given based on a method using “disk roll” as disk guides.
The above flaw which cannot be removed is called “disk mark”, it is caused not only on the surface of stainless steel and of high-alloy steel but also on the surface of carbon steel depending on the piercing-rolling condition. As a countermeasure of disk mark, a method for coating anti-seizure agent on a billet like stainless steel before heating or a method for coating a lubricant on the concave surface of the disk rolls are proposed.
As one of the examples of the above art, Patent document 1 discloses a lubricant for hot pipe rolling, in which 2 to 100 parts by mass of one or two kinds of Al2O3 and MgO as well as 2 to 10 parts by mass of SiO2 are mixed to 100 parts by mass of iron oxide. In addition, it shows that a binder is preferably mixed such that viscosity of the lubricant at room temperature be 8000 centipoise or less.
Moreover, Patent document 2 discloses a method for inhibiting guide shoe flaws generated during hot pipe rolling characterized in the steps of coating a lubricant on the outer surface of a billet in advance, and feeding the billet into a rolling mill having guide shoes.
The second type of flaw is the one which is removable by certain treatment; the flaw is generated, for example, when a material having low-deformability at high temperature such as stainless steel and high-alloy steel is worked. Majority of the flaws have a depth of less than 1 mm. Nevertheless, the flaws are generated all over the outer surface of the material, therefore removal of the flaws require enormous manpower. As a measure for inhibiting the flaws, a method is disclosed, only in Patent document 3, for keeping the temperature of material surface as high as possible by stoppage of roll cooling water, and so on.
Patent Document 1: Japanese Patent Application Examined No. 7-45056
Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 60-184410
Patent Document 3: JP-A No. 9-271811 (particularly, paragraphs [0064] to [0068])